For distributing HDR (English acronym of “High Dynamic Range”) video content, it is known to first map the HDR video content to obtain a video content of lower dynamic range, also referred to as a SDR (English acronym of “Standard Dynamic Range”) video content. The document ISO/IEC JTC1/SC29/WG11 MPEG2015/M37285 entitled Candidate Test Model for HEVC extension for HDR and WCG video coding and published in October 2015 discloses a method for encoding HDR and also Wide Gamut video contents. FIG. 1 depicts a simplified flowchart of the method for encoding a HDR picture as disclosed in the document M37285. The method begins at step S100. At step S102, a HDR picture is accessed. At step S104, the accessed HDR picture is mapped to obtain a SDR picture representation of the HDR picture, i.e. its dynamic range is reduced. In step S106, the obtained SDR picture is color corrected. The color correction, based on a luma-dependent chroma scaling, aims at controlling the color shift due to the previous mapping step. It modifies each chroma sample. In step S108, the color corrected SDR picture is encoded for example using a HEVC encoder.
FIG. 2 depicts a flowchart of a method for mapping the HDR picture to obtain a SDR picture as disclosed in the document M37285. This method may be used in the step S104 of the method depicts on FIG. 1.
In step S1042, an inverse EOTF function (EOTF stands for “Electro-Optical Transfer Function”) is applied on the HDR picture to obtain a non-linear HDR signal. In step S1044, the non-linear HDR signal is color transformed, if represented with RGB color values, to obtain a YUV signal (i.e. a signal in a YCbCr color space). In step S1046, the YUV signal is transformed from 444 to 420 format, i.e. the chroma is downsampled. In step S1048, the 420 YUV values are quantized into integer values. In step S1050, an ATF function (ATF stands for “Adaptive Transfer Functionality”) is applied on Y. ATF functionality aims at adjusting dynamic range in the YCbCr domain.
FIG. 3 depicts a flowchart of a method for color correcting the SDR picture. This method may be used in step S106 of the method depicts on FIG. 1. In a step S1060, two mono-dimensional functions β0,U and β0,V are obtained or determined. As an example, β0,U and β0,V are represented using Look-Up Tables (LUTs). These functions/parameters make it possible to control the color saturation of the SDR signal. Possibly, only one function is used for the two chroma components. In the document M37285, the functions β0,U and β0,V are referred to as βCb and βCr respectively. In step S1062, the chroma samples U and V of the SDR picture are modified according to the following equations: U=u/β0,U[Y] and V=v/β0,v[Y]. Color correction divides each Chroma component Cb and Cr, noted U and V here, respectively by a factor β0,U[Ycoloc] and β0,V[Ycoloc] which depend on the luma signal. More precisely, the luma signal considered here is the luma component value at a spatial position that is co-located with the considered Chroma sample. It is noted Ycoloc in the following. In the case of a 420 signal, the luma component may be averaged on 4 samples to find the spatial position that is co-located with the considered Chroma sample. The functions β0,U( ) and β0,V( ) may be modeled using 2 look-up-tables of dimension (2BitDepthY−1) (BitDepthY being the bit-depth of the luma samples). They can be obtained using manual tuning, to enable artistic control of the SDR rendering.
There is thus a need to determine the β0,U( ) and β0,V( ) so that the SDR picture is perceptually of good quality.